The present invention relates generally to an apparatus and method for collecting particles suspended in a gas and, more particularly, to a sampling apparatus and method for collecting particulate matter for counting and analysis.
The detection of airborne particulate matter, including fibers, pollen, mold and fungal spores, insect parts, flora and other bioaerosols, and the like, is a continuing and expanding area of development for minimizing health risks to populations. Environmental professionals need to determine the presence and quantity of deleterious particles, such as asbestos fibers, in the air. Aerobiologists and allergists need to identify and quantify airborne pollen and mold spore concentrations for patient diagnosis. Epidemiologists are concerned with particles carrying bacteria, such as that responsible for Legionnaires Disease in air conditioning systems. Moreover, federal and industrial standards have been established for allowable concentrations of particular matter in the atmosphere of various environments. As a result, it is necessary to regularly test some environments to determine the concentration of particles in the atmosphere for maintaining a particular standard or self-regulating quality control.
Devices for sampling airborne particulate matter generally include a housing having inlet and outlet openings, a pump for drawing a gas flow through the housing, and a separator within the housing for collecting particles from the sampled gas. In a conventional sampling device, referred to as an “impactor”, the separator is a flat “impaction plate”, usually a microscope slide. In use, a flow of sampled gas comprising, for example, air and particles carried by the air, is drawn through the impactor. The flow is directed through the inlet opening in the housing and toward the impaction plate. The stream of gas is diffused radially outwardly at the impaction plate surface and flows around the impaction plate. Particles in the gas stream larger than a certain size have high enough inertia to cross streamlines and impinge upon the impaction plate and are separated from the gas stream. Since the particles tend to bounce when they hit the impaction plate, the impaction plate surface is coated with an adhesive. Smaller particles remain in the gas stream and pass out of the housing through the outlet opening. Upon completion of sampling, the impaction plate is manually removed from the impactor for microscopic inspection, weighing or chemical analysis of the collected particles.
Collection efficiency of an impactor is a measure of the percent of particles which are collected on the impaction plate as a function of the particle size. The collection efficiency is usually reported as the smallest particle collected at 50% efficiency. This is known within the art as the 50% cut-off size (d50). The size range of the particles collected on the impaction plate, and the d50, is a function of the diameter of the inlet opening and the distance of the impaction plate from the opening, which is referred to as the jet-to-plate distance. These parameters are reported as a dimensionless ratio, S/W, where S is the jet-to-plate distance and W is the diameter of the inlet opening of the impactor. The 50% cut-off size is dependent upon S/W. Generally, as S/W decreases, the impactor's collection efficiency of smaller particles increases.
Collection of smaller airborne particulate such as mold and fungal spores and other bioaerosols has recently become a priority. Efficient mold and fungal spore collection requires a sampling device with a d50 of less than about 2 μm. To achieve this collection efficiency, the tendency is to reduce the S/W of the impactor. In practice, however, when S/W is less than one, conventional impactor performance becomes unpredictable yielding inconsistent results. Thus, it has been suggested that the minimum jet-to-plate distance for an impactor should provide an S/W equal to one or greater. In this configuration, small variations in jet-to-plate distance will not effect the value of d50. Unfortunately, impactors designed and operated according to these accepted parameters cannot efficiently collect particles below about 2.5 μm. and thus are inadequate for smaller particulate collection.
Another important characteristic of impactors is the gas sampling flow rate. The flow rate through the impactor must be calibrated prior to sampling in order to accurately calculate the sampling results. With conventional impactors, the flow rate through the pump is typically calibrated using a rotameter upstream of the pump. However, because the pump is spatially removed from the actual particle collection site at the impaction plate, the calibrated flow rate at the pump may not be the same as the flow rate at the point of impaction. This can lead to inaccurate sampling results.
For the foregoing reasons, there is a need for a particle collection apparatus and method for the collection of airborne particulate below about 2.5 μm. The new apparatus should be designed for sampling airborne particles in various environments and applications, including environmental air quality, industrial and occupational monitoring. The new apparatus should also allow for accurate calibration of the gas sampling flow rate at the point of impaction.